sustainability

Article

Moisture Content Impact on Properties of Briquette Producedfrom Rice Husk Waste

Anwar Ameen Hezam Saeed 1 , Noorfidza Yub Harun 1,* , Muhammad Roil Bilad 2 , Muhammad T. Afzal 3,Ashak Mahmud Parvez 3,4, Farah Amelia Shahirah Roslan 1 , Syahirah Abdul Rahim 1,Vimmal Desiga Vinayagam 1 and Haruna Kolawole Afolabi 1

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Citation: Saeed, A.A.H.; Yub Harun,

N.; Bilad, M.R.; Afzal, M.T.; Parvez,

A.M.; Roslan, F.A.S.; Abdul Rahim, S.;

Vinayagam, V.D.; Afolabi, H.K.

Moisture Content Impact on

Properties of Briquette Produced

from Rice Husk Waste. Sustainability

2021, 13, 3069. https://doi.org/

10.3390/su13063069

Academic Editor: Jun-Ichi Sakagami

Received: 5 January 2021

Accepted: 8 February 2021

Published: 11 March 2021

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4.0/).

1 Department of Chemical Engineering, Universiti Teknologi PETRONAS, Seri Iskandar 31750, Perak, Malaysia;anwar_17006829@utp.edu.my (A.A.H.S.); farah_20000280@utp.edu.my (F.A.S.R.);syahirah_19000130@utp.edu.my (S.A.R.); Vimmal_16002431@utp.edu.my (V.D.V.);haruna_18002856@utp.edu.my (H.K.A.)

2 Department of Chemistry Education, Universitas Pendidikan Mandalika (UNDIKMA), Jl. Pemuda No. 59A,Mataram 83126, Indonesia; muhammadroilbilad@ikipmataram.ac.id

3 Department of Mechanical Engineering, University of New Brunswick, 15 Dineen Drive,Fredericton, NB E3B 5A3, Canada; mafzal@unb.ca (M.T.A.); Ashak.Parvez@unb.ca (A.M.P.)

4 Institute of Combustion and Power Plant Technology (IFK), University of Stuttgart, Pfaffenwaldring 23,D-70569 Stuttgart, Germany

* Correspondence: noorfidza.yub@utp.edu.my

Abstract: An agricultural waste-based source of energy in the form of briquettes from rice husk hasemerged as an alternative energy source. However, rice husk-based briquette has a low bulk densityand moisture content, resulting in low durability. This study investigated the effect of initial moisturecontents of 12%, 14%, and 16% of rice husk-based briquettes blended with 10 wt% of kraft lignin ontheir chemical and physical characteristics. The briquetting was done using a hand push manualdie compressor. The briquette properties were evaluated by performing chemical (ultimate andproximate analysis, thermogravimetric analysis), physical (density, durability, compressive strength,and surface morphology) analyses. The durability values of all briquette samples were above 95%,meeting the standard with good compressive strength, surface morphology, and acceptable densityrange. The briquette made from the blend with 14% moisture content showed the highest calorificvalue of 17.688 MJ kg−1, thanks to its desirable morphology and good porosity range, which facilitatesthe transport of air for combustion. Overall, this study proved the approach of enhancing the qualityof briquettes from rice husk by controlling the moisture content.

Keywords: biomass briquette; agricultural waste; energy; moisture content; rice husk; lignin; blend

1. Introduction

Supplying the demanded energy has been a serious problem for many developingcountries due to set-off improvised technology and increasing population [1]. This pro-motes the utilization of multi-source fuels, particularly from sustainable and eco-friendlyresources, including biomass. Research on biomass utilization as a fuel focuses on theutilization of biomass mainly form agro-waste for producing a solid fuel block calleda briquette. It is considered as one of the available options to off-set the widening gapbetween the demand and the supply of energy, and considering the depleting nature ofnon-renewable energy sources from fossil fuels [2,3].

As the major Malaysian food crop, the annual production of rice husk, a side product of paddymill processing, recorded by the Malaysia Ministry of Agriculture is 408,000 metric tons [4,5]. Ricehusk is the outer layer of rice grain obtained from the paddy milling process [6]. Theproduct obtained from rice consists of 72% grain, 5–8% bran, and 20–22% rice husk [7].Generally, rice husk consists of 40% lignocellulose and cellulose, 5% hemicellulose as five-carbon sugar polymers, and the rest is minerals such as silica, alkali, and trace elements [8].

Sustainability 2021, 13, 3069. https://doi.org/10.3390/su13063069 https://www.mdpi.com/journal/sustainability

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Table 1 shows a typical analysis of rice husk, as they vary according to climate condition,soil chemistry, and even the type of rice produced [9]. Rice husk is often dumped onlandfills, which causes storage and operation problems. Additionally, poor managementof rice husk causes serious environmental problems, as it is typically burned in openair [10,11].

Table 1. Typical analysis of rice husk [12].

Property Range Basis

Bulk density (kg/m3) 96–160 dryLength of the husk (mm) 2.0–5.0 dryHardness (Mohr’s scale) 5.0–6.0 dry

Ash (%) 18–29 dryCarbon (%) 35.0–42.0 dry

Hydrogen (%) 4.0–5.0 dryOxygen (%) 31.0–37.0 dry

Nitrogen (%) 0.23–0.32 drySulphur (%) 0.04–0.08 dry

Moisture content (%) 6.0–10.0 As received

The use of rice husk as a base material for fuel briquette production has been shown tobe effective in overcoming its negative impacts [13]. However, rice husk with low moisturecontent fails to produce a durable briquette. Thus, the high ash content becomes cloggedin the boiler system during combustion, resulting in equipment failure and a high costof maintenance. Moreover, agricultural residue-based briquette possesses a low quality,due to low lignin content [14,15]. The use of the bulk of biomass residue as fuel canminimize the wastage of valuable products by obeying the 3R concept (reduce, reuse, andrecycle) [16].

The quality of a briquette greatly depends on its physical and chemical properties,which dictate its durability and its energy value [17]. Processing variables such as pressure,temperature, particle size, binding ratio, and moisture content massively affect the qualityof the resulting briquettes [18,19]. This study explores the effect of moisture content on theresulting quality and durability of a briquette produced from rice husk. Briquettes wereproduced from rice husks with moisture contents of 12%, 14%, and 16%. Since rice huskwas found to be low in moisture content and lignin, a kraft lignin binder was used to easethe compaction of the rice husk briquette [20,21]. After production, the rice husk briquetteswere further characterized in terms of physical and chemical properties, with a focus onenergy content (heating value) and standard durability. This study investigates the effect ofinitial moisture contents of 12%, 14%, and 16% of rice husk-based briquette blended with10 wt% of kraft lignin on their chemical and physical characteristics, and demonstrates thatrice husk has potential to be converted into highly durable briquettes.

2. Materials and Methods2.1. Sample Preparation

The rice husk was obtained from the Bernas Kedah paddy mill, and the kraft ligninwas obtained from Lembaga Getah, Malaysia. The raw rice husk was stored in a closedcontainer under ambient temperature to prevent contamination. First, the rice husk wasground using an auto-grinder, followed by sieving with a mesh size of 500 µm. Eachbriquette was made from 10 g of blends, with a ratio of 90:10, and at different moisturecontents of 12%, 14%, and 16%. The blending was done mechanically for 15 min. Themoisture content of the blended biomass was measured using a Metler Toledo moisturecontent analyzer. The excess water content in the blend was removed by drying in an ovenat a temperature of 105 ◦C, and the moisture drop was recorded regularly until the desiredconcentration was obtained. When the initial moisture content was lower than that aimedat, liquid water was sprayed in excess on the blend, and the process of adjusting moisturecontent via drying was performed until reaching the desired concentration. The blends

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with desired moisture contents were then placed in a closed container to maintain theirmoisture content before processing.

2.2. Briquetting

The briquette samples were prepared in the form of a standard die at a diameter of 4cm. The pressure was set constant at 150 MPa with 120 s holding time, and the briquettingwas done at room temperature, 25 ◦C. The pressure level was selected based on a previousbiomass briquetting study, in which the minimum pressure required was 150–200 MPa.The applied pressure, coupled with a proper moisture content feed, resulted in a briquettewith desirable durability. Most researchers found that higher pressure tends to generateheat at elevated temperature from the die, which significantly melts the lignin content,whereby better compaction can be expected [22]. The compaction process was performedby using a hand-push manual pressure-based die compressor. The produced briquetteswere stored in a closed container until further characterizations.

2.3. Physical Properties Characterization2.3.1. Density

The rice husk blend density was examined before and after briquetting to observethe density change. Three density parameters were tested which were tapped density,maximum density, and relaxed density. Tapped density was studied by inserting the ricehusk blend in a cylindrical measuring tube and measuring the linear dimension (diameterand thickness) using a vernier caliper. The density was then calculated. The maximumdensity was calculated after the ejection of the briquette from the die compressor. Similarly,the relaxed density was measured after 7 days from the ejection day.

2.3.2. Durability

A drop test was used to examine the briquette durability. The rice husk briquettes weredropped vertically from a height of 2 m, then the mass loss was weighed. The durabilitywas calculated using Equation (1) [23]. This study is crucial, as it determines the qualityand handling of briquette in terms of storage, transportation, and operation.

Durability =Mass o f briquette be f ore drop test

Mass o f briquette a f ter the drop test(1)

2.3.3. Compressive Strength

The compressive strength of rice husk briquette was tested according to the ASTMD575 method, using a universal testing machine [24]. A maximum load of 5 kN was applieddiametrically on the briquette samples. This test was performed to observe the maximumload a briquette can withstand before any cracks or breaks appeared. The presence of thekraft lignin binder eased the compaction, and a detailed method on the preparation of thekraft lignin binder is available elsewhere [20].

2.3.4. Surface Morphology

The surface morphology was obtained using scanning electron microscopy, mainly toobserve the surface of the briquette in terms of a failure of the briquette after the maximumload was applied. Prior to measurement, the sample was dried at room temperature andcoated with gold to avoid overcharging during the analysis at 15 kV. The contact betweeneach particle can be observed through SEM testing, and the morphology can be used tojustify the finest durability of the briquette sample.

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2.4. Chemical Properties characterization2.4.1. Ultimate Analysis and Proximate Analysis

The ultimate analysis of both the rice husk blend and the briquette was as determinedby the element analyzer (CHNS analyzer ASTM D5291 Method) [24]. The proximate analy-sis of the rice husk blend and briquette was determined mainly on the higher heating value(HHV), ash content, and the presence of volatile matter, fixed carbon, and moisture contentof the briquette. The higher heating value was determined by droperidol calorimeter. Theash content was determined by burning 2 g of briquette sample in a crucible placed in afurnace at a temperature of 700 ◦C for 3 h. The ash percentage (Ash %) was calculated byusing Equation (2).

Ash% =y(g) − x(g)

m(g)× 100% (2)

where y is the mass of the crucible lid with ash sample after analysis (g), x is the initialmass of the crucible lid (g), and m is the mass of the sample (g).

2.4.2. Thermogravimetric Analysis

The samples were heated until the maximum temperature of 800 ◦C with a heating rateof 20 ◦C/min. The degradation temperature of the briquette was identified through TGAand derivative thermogravimetric analysis (DTG) analysis. The degradation temperatureis defined as the temperature with maximum weight loss. Subsequently, the fixed carbon,the volatile matter, and the moisture content of the rice husk blend and the briquette weredetermined from the TGA plot.

3. Results and Discussion3.1. Briquette Compression

Figure 1 shows the rice husk briquette formed at various blend moisture contents,of 12%, 14%, and 16%. Since the particle size of the rice husk was less than 500 µm,the texture of the briquette obtained was smooth, which gives a better appearance [25].The combinations of materials, blend ratio, applied pressure, and biomass size, as seenin Figure 1, gave the briquette samples a good form. A similar method of briquettecompression was done by Ndindeng et al. [26], in which the briquetting was performed ata moisture content of 20%, usually after 7 days, for samples dried in the oven, and 21 daysfor samples dried under ambient conditions [27].

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between each particle can be observed through SEM testing, and the morphology can be

used to justify the finest durability of the briquette sample.

2.4. Chemical Properties characterization

2.4.1. Ultimate Analysis and Proximate Analysis

The ultimate analysis of both the rice husk blend and the briquette was as determined

by the element analyzer (CHNS analyzer ASTM D5291 Method) [24]. The proximate anal-

ysis of the rice husk blend and briquette was determined mainly on the higher heating

value (HHV), ash content, and the presence of volatile matter, fixed carbon, and moisture

content of the briquette. The higher heating value was determined by droperidol calorim-

eter. The ash content was determined by burning 2 g of briquette sample in a crucible

placed in a furnace at a temperature of 700 °C for 3 h. The ash percentage (Ash %) was

calculated by using Equation (2).

Ash% = y(g) − x(g)

m(g) × 100% (2)

where y is the mass of the crucible lid with ash sample after analysis (g), x is the initial

mass of the crucible lid (g), and m is the mass of the sample (g).

2.4.2. Thermogravimetric Analysis

The samples were heated until the maximum temperature of 800 °C with a heating

rate of 20 °C/min. The degradation temperature of the briquette was identified through

TGA and derivative thermogravimetric analysis (DTG) analysis. The degradation temper-

ature is defined as the temperature with maximum weight loss. Subsequently, the fixed

carbon, the volatile matter, and the moisture content of the rice husk blend and the bri-

quette were determined from the TGA plot.

3. Results and Discussion

3.1. Briquette Compression

Figure 1 shows the rice husk briquette formed at various blend moisture contents, of

12%, 14%, and 16%. Since the particle size of the rice husk was less than 500 µm, the tex-

ture of the briquette obtained was smooth, which gives a better appearance [25]. The com-

binations of materials, blend ratio, applied pressure, and biomass size, as seen in Figure

1, gave the briquette samples a good form. A similar method of briquette compression

was done by Ndindeng et al. [26], in which the briquetting was performed at a moisture

content of 20%, usually after 7 days, for samples dried in the oven, and 21 days for samples

dried under ambient conditions [27].

Figure 1. Pictures of the briquettes produced from rice husk at various moisture contents of (a)

12%, (b) 14%, and (c) 16%.

3.2. Physical Properties

3.2.1. Density

Figure 1. Pictures of the briquettes produced from rice husk at various moisture contents of (a) 12%,(b) 14%, and (c) 16%.

3.2. Physical Properties3.2.1. Density

The properties of raw blends and briquettes at various moisture contents are summa-rized in Table 2. In general, the moisture content affects the density, the angle of friction, theforce deformation characteristics, and the thermal conductivity, as reported elsewhere [28].Based on the results obtained, the tapped density increased slightly at higher moisturecontents. This can be attributed to the moisture contained in the blend. The maximumdensity obtained was calculated after the ejection of the briquette from the hand-push

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manual compressor after 120 s of holding time. The briquette with maximum density wasobtained with moisture content 16%, corresponding to a density of 1144 ± 0.63 kg/m3,followed by 14% and 12%, with 1141 ± 0.22 kg/m3 and 1136 ± 0.67 kg/m3, respectively.The moisture content filled the micro gap in between the rice husk particles, connected bythe kraft lignin binder. At high pressure, heat is generated at elevated temperatures in thedie, which melts the binding agent and increases the compactness of the resulting briquette.Therefore, there is the possibility of fusing lignin more in the presence of higher moisture.

Table 2. Rice husk blend and briquette densities at various moisture contents.

Moisture Content(%)

Tapped Density(kg/m3)

Maximum Density(kg/m3)

Relaxed Density(kg/m3)

12% 144 ± 0.66 1136 ± 0.67 1136 ± 0.6714% 144 ± 0.67 1141 ± 0.22 825 ± 0.2616% 144 ± 0.71 1144 ± 0.63 673.± 0.17

The relaxed density data show that the briquette made from the blend with themoisture content of 12% showed the highest density, followed by 14% and 16%, of825 ± 0.26 kg/m3 and 673.17 ± 0.17 kg/m3, respectively. According to Mohsenin et al. [29],higher feedstock moisture content allows the briquette to expand, due to the higher residualstress after briquette ejection, causing elastic rebound or enlargement. A similar justifica-tion is applied in this study, as the density decreased with increasing moisture content.Moreover, a smaller particle size of biomass residue promotes a higher density underhigh pressures, as reported earlier [29]. The density of briquette increases with increasingmoisture content until it reaches the maximum, and further increase in moisture contenttends to reduce the density of briquette [25,30]. The standard briquette density falls withinthe range of 750 kg/m3 and above so, based on these findings, briquette samples preparedfrom blends with moisture contents of 12% and 14% were identified as suitable, as theymeet the standard briquette density requirement.High moisture content is able to makebriquettes expand due to higher residual stress after ejection, which typically causes elasticrebound or enlargement [29,31].

3.2.2. Durability

Figure 2 shows durability data of the briquettes prepared from a blend with moisturecontents of 12%, 14%, and 16% over 7 days of storage. They vary accordingly betweenintervals from day 0 until day 7. Durability percentage at moisture content 12% and 14%dropped only from 100% to 99%, and 100% to 98%, respectively, over 7 days, suggestingtheir long-term stability. However, the durability of the briquette from a blend witha moisture content of 16% dropped 1% in each day from day 0 to day 7, which can beattributed to the high moisture content in the briquette, which tends to reduce the durability,even though it enhances the burning efficiency and life span [32]. Typically, feedstockmoisture content benefits the intermolecular and interfacial force which binds the particleby pressure and temperature generated by the die when compressed at high pressures [33].The binding agent helps in fusing the biomass particles, and thus enhances the durabilityof the resulting briquette. Therefore, the durability of rice husk briquette at all moisturecontents was above 96%, which falls within the standard pellet quality 95–100% durabilityrange [34]. The results show that the standard durability of the biomass briquettes fellwithin the range of 95% and above. Moisture content is the main process parameter thatsignificantly affects the durability. Most researchers found that lower durability oftenresults in fines and hinders the operation and transportation.

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that significantly affects the durability. Most researchers found that lower durability often

results in fines and hinders the operation and transportation.

Figure 2. Durability study of rice husk briquette at various moisture contents.

3.2.3. Compressive Strength

The data on compressive strength summarized in Table 3 indicate that the produced

briquette samples survived any deformation, either in the form of fracture or breaking,

under the maximum load of 5 kn, as applied by the universal testing machine. This supe-

rior compressive strength can be attributed to the high applied pressure of 150 MPa dur-

ing the compression stage. The maximum load applied in the study was also high, sug-

gesting the high quality of the produced briquettes. The results suggest that the produced

briquette can be labelled as high durability with good compressive strength, which was

also reported from another study for briquette formation using rice husk [35]. The data of

the compression test in Table 3 do not clearly show the relative strength of the briquettes

prepared from different moisture contents. All sample could withstand the load of 500

kgf. However, one can deduce the relative strength of the samples from other properties,

such as the pore volume and porosity, as detailed in another report [36].

Table 3. Compressive strength study for rice husk briquette at various moisture contents.

Length (mm) Thickness (mm) Load @ 8 mm (kgf)

12% 40 9 500

14% 40 9 500

16% 40 9 500

3.2.4. Surface Morphology

The surface morphologies of the briquette samples prepared from a blend under var-

ious moisture contents are shown in Figure 3. SEM images are useful to observe the bind-

ing of particles, and voids/pores formed in the briquette structure. Figure 3 shows that the

bonding between the particles was mainly solid bridges. It was reported earlier that the

bonding force between the particles was affected by the process variables of a standard

biomass solid, such as moisture content, binder, temperature, and pressure [37]. Accord-

ing to the microstructure images in Figure 3, there was no obvious pores on the surface.

However, as shown from the highest heating value (HHV) measurements, the briquette

prepared from the blend at a moisture content of 14% is the most attractive. We believe

0%

20%

40%

60%

80%

100%

Day 0 Day 3 Day 5 Day 7

Du

rab

ility

Time (days)

12% 14% 16%

Figure 2. Durability study of rice husk briquette at various moisture contents.

3.2.3. Compressive Strength

The data on compressive strength summarized in Table 3 indicate that the producedbriquette samples survived any deformation, either in the form of fracture or breaking,under the maximum load of 5 kn, as applied by the universal testing machine. Thissuperior compressive strength can be attributed to the high applied pressure of 150 MPaduring the compression stage. The maximum load applied in the study was also high,suggesting the high quality of the produced briquettes. The results suggest that theproduced briquette can be labelled as high durability with good compressive strength,which was also reported from another study for briquette formation using rice husk [35].The data of the compression test in Table 3 do not clearly show the relative strength ofthe briquettes prepared from different moisture contents. All sample could withstand theload of 500 kgf. However, one can deduce the relative strength of the samples from otherproperties, such as the pore volume and porosity, as detailed in another report [36].

Table 3. Compressive strength study for rice husk briquette at various moisture contents.

Length (mm) Thickness (mm) Load @ 8 mm (kgf)

12% 40 9 50014% 40 9 50016% 40 9 500

3.2.4. Surface Morphology

The surface morphologies of the briquette samples prepared from a blend undervarious moisture contents are shown in Figure 3. SEM images are useful to observe thebinding of particles, and voids/pores formed in the briquette structure. Figure 3 showsthat the bonding between the particles was mainly solid bridges. It was reported earlierthat the bonding force between the particles was affected by the process variables of astandard biomass solid, such as moisture content, binder, temperature, and pressure [37].According to the microstructure images in Figure 3, there was no obvious pores on thesurface. However, as shown from the highest heating value (HHV) measurements, thebriquette prepared from the blend at a moisture content of 14% is the most attractive. Webelieve that it possesses an optimum pore size, which allows the oxygen from the air todiffuse through the pore and sustain better combustion, with longer ignition compared tothe rest.

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that it possesses an optimum pore size, which allows the oxygen from the air to diffuse

through the pore and sustain better combustion, with longer ignition compared to the

rest.

Figure 3. Surface morphology of rice husk briquette: (a) 12%, (b) 14%, and (c) 16% moisture content.

3.3. Chemical Properties

3.3.1. Proximate and Ultimate Analysis

The rice husk briquette at various moisture contents showed different elemental com-

positions, as the moisture content influenced the composition, as shown in Table 4. Rice

husk blend showed a lower elemental percentage compared to rice husk briquette, mainly

in carbon content, which was the most crucial element, as it affected combustion behavior.

According to biomass residues it contained a high amount of carbon and hydrogen. The

elemental compositions and the HHV changed substantially when compared to the raw

rice husk with the blend rice husk. Those changes can be attributed to the different com-

positions and the HHV of the kraft lignin used as the binder, which accounted for 10% wt.

Analysis of the composition and HHV of the blend and the briquette shows that they did

not differ significantly, and suggesting that no reaction occurred during mixing, storage,

and briquetting that could affect the material composition and hence the HHV.

Table 4. The ultimate and proximate analysis of rice husk blend and briquette at a various mois-

ture contents.

Ultimate Analysis Proximate Analysis

MC C

(%)

H

(%)

N

(%)

S

(%)

O

(%) VM (%) FC (%)

ASH

(%)

HHV

(MJ/kg)

10%

Raw RH 35.82 6.15 5.57 0.52 51.95 nd nd nd 8.97

Rice Husk Blend

12% 36.63 5.89 5.96 0.82 50.70 40 45.53 4.92 13.87

14% 34.46 5.55 5.63 1.00 53.33 30 55.93 4.70 14.23

16% 34.42 5.54 5.93 1.17 52.94 39 47.73 5.27 13.08

Rice Husk Briquette

12% 39.43 5.87 6.36 0.81 48.43 42.00 45.79 4.21 14.04

14% 41.23 5.71 5.81 0.73 46.55 40.50 46.00 3.97 17.69

16% 38.76 5.99 5.54 0.78 49.00 41.00 46.37 4.63 13.10

MC = moisture content, C = carbon, H = hydrogen, N = nitrogen, S = sulphur, O = oxygen, VM =

volatile matter, FC = fixed carbon, HHV = higher heating value, RH = rice husk, nd = no detectable.

Figure 3. Surface morphology of rice husk briquette: (a) 12%, (b) 14%, and (c) 16% moisture content.

3.3. Chemical Properties3.3.1. Proximate and Ultimate Analysis

The rice husk briquette at various moisture contents showed different elementalcompositions, as the moisture content influenced the composition, as shown in Table 4. Ricehusk blend showed a lower elemental percentage compared to rice husk briquette, mainlyin carbon content, which was the most crucial element, as it affected combustion behavior.According to biomass residues it contained a high amount of carbon and hydrogen. Theelemental compositions and the HHV changed substantially when compared to the rawrice husk with the blend rice husk. Those changes can be attributed to the differentcompositions and the HHV of the kraft lignin used as the binder, which accounted for10% wt. Analysis of the composition and HHV of the blend and the briquette shows thatthey did not differ significantly, and suggesting that no reaction occurred during mixing,storage, and briquetting that could affect the material composition and hence the HHV.

Table 4. The ultimate and proximate analysis of rice husk blend and briquette at a various moisturecontents.

Ultimate Analysis Proximate Analysis

MC C(%)

H(%)

N(%)

S(%)

O(%)

VM(%)

FC(%)

ASH(%)

HHV(MJ/kg)

10%RawRH

35.82 6.15 5.57 0.52 51.95 nd nd nd 8.97

Rice Husk Blend

12% 36.63 5.89 5.96 0.82 50.70 40 45.53 4.92 13.8714% 34.46 5.55 5.63 1.00 53.33 30 55.93 4.70 14.2316% 34.42 5.54 5.93 1.17 52.94 39 47.73 5.27 13.08

Rice Husk Briquette

12% 39.43 5.87 6.36 0.81 48.43 42.00 45.79 4.21 14.0414% 41.23 5.71 5.81 0.73 46.55 40.50 46.00 3.97 17.6916% 38.76 5.99 5.54 0.78 49.00 41.00 46.37 4.63 13.10

MC = moisture content, C = carbon, H = hydrogen, N = nitrogen, S = sulphur, O = oxygen, VM = volatile matter,FC = fixed carbon, HHV = higher heating value, RH = rice husk, nd = no detectable.

Generally, high carbon content influences the combustion behavior, which affects theash fusion [38]. Under the proximate analysis study, ash content was the major concern, as

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it is one of the limitations of using rice husk as a solid fuel. Ash content is predominantinorganic substance obtained after complete combustion of biomass takes place [39]. Typi-cally, ash content contains calcium, potassium, magnesium, and phosphorus. One problemthat hinders the thermal process of biomass is the deposition and agglomeration causedby minerals in ash melts [40]. Therefore, a higher fixed carbon content promotes highercalorific energy in the charcoal produced. Additionally, a higher fixed carbon contentresults in a lower volatile matter content. The overall mass of the briquette decreases as thevolatile combustion phase takes place. Increasing hydrogen to carbon ratio augmented thecombustion, although to a lesser extent, as the carbon ratio rises. However, biomass withhigh volatile content is able to lose 90% of its initial mass at the devolatilization stage ofcombustion. Hence, from the proximate and ultimate analysis obtained it was crucial tomake a comparison of other studies that have been done with this project, mainly of thecalorific value and thermogravimetric analysis [41].

3.3.2. Calorific Value

Figure 4 shows the higher heating value of rice husk blend and briquette at variousmoisture contents. At moisture content 14%, the highest heating values were 14.23 MJ/kgbefore briquetting, and 17.69 MJ/kg after briquetting. While at the moisture contentsof 12% and 16% the heating value raised significantly from 13.87 MJ/kg to 14.04 MJ/kgand 13.08 MJ/kg to 13.11 MJ/kg, respectively. Since the rice husk briquette at a moisturecontent of 14% produced the highest heating value, it was selected as the best rice huskbriquette developed in this study.

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Generally, high carbon content influences the combustion behavior, which affects the

ash fusion [38]. Under the proximate analysis study, ash content was the major concern,

as it is one of the limitations of using rice husk as a solid fuel. Ash content is predominant

inorganic substance obtained after complete combustion of biomass takes place [39]. Typ-

ically, ash content contains calcium, potassium, magnesium, and phosphorus. One prob-

lem that hinders the thermal process of biomass is the deposition and agglomeration

caused by minerals in ash melts [40]. Therefore, a higher fixed carbon content promotes

higher calorific energy in the charcoal produced. Additionally, a higher fixed carbon con-

tent results in a lower volatile matter content. The overall mass of the briquette decreases

as the volatile combustion phase takes place. Increasing hydrogen to carbon ratio aug-

mented the combustion, although to a lesser extent, as the carbon ratio rises. However,

biomass with high volatile content is able to lose 90% of its initial mass at the devolatiliza-

tion stage of combustion. Hence, from the proximate and ultimate analysis obtained it was

crucial to make a comparison of other studies that have been done with this project,

mainly of the calorific value and thermogravimetric analysis [41].

3.3.2. Calorific Value

Figure 4 shows the higher heating value of rice husk blend and briquette at various

moisture contents. At moisture content 14%, the highest heating values were 14.23 MJ/kg

before briquetting, and 17.69 MJ/kg after briquetting. While at the moisture contents of

12% and 16% the heating value raised significantly from 13.87 MJ/kg to 14.04 MJ/kg and

13.08 MJ/kg to 13.11 MJ/kg, respectively. Since the rice husk briquette at a moisture con-

tent of 14% produced the highest heating value, it was selected as the best rice husk bri-

quette developed in this study.

Figure 4. The higher heating value of rice husk blend and briquette at a various moisture contents.

The heating value increases as the wet basis of the briquette reduces, while the mois-

ture content must be at an optimum level for the briquette to have good combustive be-

havior [42]. High carbon and hydrogen component presence promotes a better heating

value, while inversely a high ash content gives a lower heating value. Besides that, the

binding agent may typically influence the incremental heating value [43]. The best dura-

bility also gives a significantly higher heating value when combined with optimum pore

volume, which allows the gases to pass through the pore to supply oxygen for the com-

bustion reaction. Consequently, all the briquettes produced at various moisture contents

were able to promote their respective heating values according to their properties.

3.3.3. Thermogravimetric Analysis

0

4

8

12

16

20

12% 14% 16%

Hig

her

Hea

tin

g V

alu

e (M

J/K

g)

Moisture content (%)

Blended Biomass Briquette

Figure 4. The higher heating value of rice husk blend and briquette at a various moisture contents.

The heating value increases as the wet basis of the briquette reduces, while the mois-ture content must be at an optimum level for the briquette to have good combustivebehavior [42]. High carbon and hydrogen component presence promotes a better heatingvalue, while inversely a high ash content gives a lower heating value. Besides that, the bind-ing agent may typically influence the incremental heating value [43]. The best durabilityalso gives a significantly higher heating value when combined with optimum pore volume,which allows the gases to pass through the pore to supply oxygen for the combustionreaction. Consequently, all the briquettes produced at various moisture contents were ableto promote their respective heating values according to their properties.

3.3.3. Thermogravimetric Analysis

Figures 5–7 show the thermogravimetric analysis graph plotted for sample temper-ature concerning weight percentage and derivative weight percentage for all rice huskblends and briquettes at various moisture contents. Thermogravimetric analysis was per-

Sustainability 2021, 13, 3069 9 of 14

formed for rice husk briquette to determine the degradation profile, without limitationin heat and mass transfer of a low heating rate. From the sample temperature versusweight percentage weight plots, the briquette volatile matter, fixed carbon, and moisturecan be determined. It can also be assigned as the degradation temperature of rice husk.Besides this, from the sample temperature versus the derivative weight percentage plot,the degradation temperature of the briquette moisture content, volatile matter, and fixedcarbon can be determined. However, the focus of this study was to determine the ricehusk blend and briquette degradation temperature. Thus, for a clear observation of thedegradation temperature, the value was extracted from the graph summarized in Table 5.

Sustainability 2021, 13, x FOR PEER REVIEW 9 of 15

Figures 5–7 show the thermogravimetric analysis graph plotted for sample tempera-

ture concerning weight percentage and derivative weight percentage for all rice husk

blends and briquettes at various moisture contents. Thermogravimetric analysis was per-

formed for rice husk briquette to determine the degradation profile, without limitation in

heat and mass transfer of a low heating rate. From the sample temperature versus weight

percentage weight plots, the briquette volatile matter, fixed carbon, and moisture can be

determined. It can also be assigned as the degradation temperature of rice husk. Besides

this, from the sample temperature versus the derivative weight percentage plot, the deg-

radation temperature of the briquette moisture content, volatile matter, and fixed carbon

can be determined. However, the focus of this study was to determine the rice husk blend

and briquette degradation temperature. Thus, for a clear observation of the degradation

temperature, the value was extracted from the graph summarized in Table 5.

(a) (b)

Figure 5. Thermogravimetric analysis (TGA) and derivative thermogravimetric analysis (DTA) analysis of (a) rice husk

blend, and (b) rice husk briquette at 12% moisture content.

(a) (b)

Figure 6. TGA and DTA analysis of (a) rice husk blend, and (b) rice husk briquette at 14% moisture content.

-6

-5

-4

-3

-2

-1

0

1

2

3

0

20

40

60

80

100

0 200 400 600 800

TG.Weight (%)

Temperature (°C)

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0

20

40

60

80

100

0 200 400 600 800

DTG:Deriv Weight (%)

TG.Weight (%)

Temperature (°C)

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

0

20

40

60

80

100

0 200 400 600 800

DTG: Deriv Weight (%)

TG Weight (%)

Temperature (°C)

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

0

20

40

60

80

100

0 200 400 600 800

DTG

.Der

iv W

eigh

t ,(

%)

TG. Weight, (%)

Temperarure (°C)

Figure 5. Thermogravimetric analysis (TGA) and derivative thermogravimetric analysis (DTA) analysis of (a) rice huskblend, and (b) rice husk briquette at 12% moisture content.

Sustainability 2021, 13, x FOR PEER REVIEW 9 of 15

Figures 5–7 show the thermogravimetric analysis graph plotted for sample tempera-

ture concerning weight percentage and derivative weight percentage for all rice husk

blends and briquettes at various moisture contents. Thermogravimetric analysis was per-

formed for rice husk briquette to determine the degradation profile, without limitation in

heat and mass transfer of a low heating rate. From the sample temperature versus weight

percentage weight plots, the briquette volatile matter, fixed carbon, and moisture can be

determined. It can also be assigned as the degradation temperature of rice husk. Besides

this, from the sample temperature versus the derivative weight percentage plot, the deg-

radation temperature of the briquette moisture content, volatile matter, and fixed carbon

can be determined. However, the focus of this study was to determine the rice husk blend

and briquette degradation temperature. Thus, for a clear observation of the degradation

temperature, the value was extracted from the graph summarized in Table 5.

(a) (b)

Figure 5. Thermogravimetric analysis (TGA) and derivative thermogravimetric analysis (DTA) analysis of (a) rice husk

blend, and (b) rice husk briquette at 12% moisture content.

(a) (b)

Figure 6. TGA and DTA analysis of (a) rice husk blend, and (b) rice husk briquette at 14% moisture content.

-6

-5

-4

-3

-2

-1

0

1

2

3

0

20

40

60

80

100

0 200 400 600 800

TG.Weight (%)

Temperature (°C)

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0

20

40

60

80

100

0 200 400 600 800

DTG:Deriv Weight (%)

TG.Weight (%)

Temperature (°C)

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

0

20

40

60

80

100

0 200 400 600 800

DTG: Deriv Weight (%)

TG Weight (%)

Temperature (°C)

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

0

20

40

60

80

100

0 200 400 600 800

DTG

.Der

iv W

eigh

t ,(

%)

TG. Weight, (%)

Temperarure (°C)

Figure 6. TGA and DTA analysis of (a) rice husk blend, and (b) rice husk briquette at 14% moisture content.

Sustainability 2021, 13, 3069 10 of 14Sustainability 2021, 13, x FOR PEER REVIEW 10 of 15

(a) (b)

Figure 7. TGA and DTA analysis of (a) rice husk blend, and (b) rice husk briquette at 16% moisture content.

Table 5. The thermogravimetric analysis of blended rice husk and briquette at various moisture

contents.

Degradation Temperature°C

Moisture Content 12% 14% 16%

Blend 330 ± 10 270 ± 10 320 ± 10

Briquette 300 ± 10 300 ± 10 300 ± 10

According to the results shown in Table 5, a small weight loss for the rice husk blends

occurred at a temperature range of 50–200 °C, and maximum weight loss occurred at a

temperature range of 250–330 °C for all samples. A rather higher range of small weight

loss was observed for the rice husk briquette at 50–220 °C, and the peak weight loss oc-

curred at a temperature of 300 °C.

Before analysis, the degradation temperature varied before rice husks were com-

pacted into briquettes, where at moisture content 14% the degradation occurred earliest

at a temperature 270 ± 10 °C, compared to the rice husk blend at moisture contents 12%

and 16%, which degraded at temperatures 330 ± 10 °C and 320 ± 10 °C, respectively. This

result indicates that lower temperatures are required for the water content of the rice husk

blend at a moisture content of 14% to start vaporizing, which promotes a better heating

value than a rice husk blend at moisture contents of 12% and 16%, as shown in Figure 4.

However, the degradation temperature for the rice husk briquettes at all moisture con-

tents was at 300 ± 10 °C. This was due to the durability of the briquettes obtained being

similar, and above the standard pellet and briquette quality [44]. The range of moisture

content difference was only 4% between all the briquettes, allowing the degradation tem-

perature to be similar. Therefore, the rice husk briquettes of all moisture contents are de-

graded at the same temperature, but the heating value differs.

3.4. Comparison with Agricultural Biomass Residue-Based Briquette

Table 6 summarizes the typical moisture content of a few agricultural residues which

have been widely used in energy applications. Moisture content in a material is affected

by many factors. Generally, biomass has two types of moisture content (intrinsic and ex-

trinsic) [45]. Intrinsic moisture content, also called bound water, is the moisture from the

biomass itself. It is not affected by external factors. On the other hand, extrinsic moisture

is the moisture which can be changed, depending on weather conditions during harvest-

ing. The moisture content of agricultural waste residues such as husk and straw falls from

below 20% to up to 60% for bagasse, while the moisture content of forest residue falls

between the range 40 and 50% [46].

As shown in Table 6, the moisture content of biomass residue varies accordingly in

different parts and catalogs. According to L. Chena et al. [47], the moisture content is best

-4.5-4-3.5-3-2.5-2-1.5-1-0.50

0

20

40

60

80

100

0 200 400 600 800

DTG: Deriv Weight (%)

TG. Weight (%)

Temperature (°C)

-5

-4

-3

-2

-1

0

1

2

10

30

50

70

90

0 200 400 600 800

DT

G: D

eriv

Wei

gh

t (%

)

TG.Weight (%)

Temperature (°C)

Figure 7. TGA and DTA analysis of (a) rice husk blend, and (b) rice husk briquette at 16% moisture content.

Table 5. The thermogravimetric analysis of blended rice husk and briquette at various moisturecontents.

Degradation Temperature◦C

Moisture Content 12% 14% 16%Blend 330 ± 10 270 ± 10 320 ± 10

Briquette 300 ± 10 300 ± 10 300 ± 10

According to the results shown in Table 5, a small weight loss for the rice husk blendsoccurred at a temperature range of 50–200 ◦C, and maximum weight loss occurred at atemperature range of 250–330 ◦C for all samples. A rather higher range of small weight losswas observed for the rice husk briquette at 50–220 ◦C, and the peak weight loss occurred ata temperature of 300 ◦C.

Before analysis, the degradation temperature varied before rice husks were compactedinto briquettes, where at moisture content 14% the degradation occurred earliest at atemperature 270 ± 10 ◦C, compared to the rice husk blend at moisture contents 12% and16%, which degraded at temperatures 330 ± 10 ◦C and 320 ± 10 ◦C, respectively. Thisresult indicates that lower temperatures are required for the water content of the rice huskblend at a moisture content of 14% to start vaporizing, which promotes a better heatingvalue than a rice husk blend at moisture contents of 12% and 16%, as shown in Figure 4.However, the degradation temperature for the rice husk briquettes at all moisture contentswas at 300 ± 10 ◦C. This was due to the durability of the briquettes obtained being similar,and above the standard pellet and briquette quality [44]. The range of moisture contentdifference was only 4% between all the briquettes, allowing the degradation temperatureto be similar. Therefore, the rice husk briquettes of all moisture contents are degraded atthe same temperature, but the heating value differs.

3.4. Comparison with Agricultural Biomass Residue-Based Briquette

Table 6 summarizes the typical moisture content of a few agricultural residues whichhave been widely used in energy applications. Moisture content in a material is affectedby many factors. Generally, biomass has two types of moisture content (intrinsic andextrinsic) [45]. Intrinsic moisture content, also called bound water, is the moisture from thebiomass itself. It is not affected by external factors. On the other hand, extrinsic moisture isthe moisture which can be changed, depending on weather conditions during harvesting.The moisture content of agricultural waste residues such as husk and straw falls frombelow 20% to up to 60% for bagasse, while the moisture content of forest residue fallsbetween the range 40 and 50% [46].

As shown in Table 6, the moisture content of biomass residue varies accordinglyin different parts and catalogs. According to L. Chena et al. [47], the moisture contentis best to be low, in a range of 10–15%, as high moisture content would require energy

Sustainability 2021, 13, 3069 11 of 14

for drying prior to being used as a fuel, which was also reported in [48]. Besides that,optimum moisture content for cellulosic biomass falls within the range of 8–12% [49].Feedstock moisture content is essential, as it is the most important variable for controllingthe moisture uptake in the production of pellets [50]. The comparison results show thatthe rice husk blend with kraft lignin is highly durable at all moisture contents, where thekraft lignin works as a binder, which makes a huge impact on the production of solid fuel.The calorific values of the samples reported in Table 6 were probably determined by thesame technology, that is a bomb calorimeter. It was found that the highest calorific values,shown as highest heating value (HHV) in Table 6, were rice bran (20.5 MJ/kg), followedby coffee husk (19.35 MJ/kg), oat straw (17.54 MJ/kg), corn cob (17.25 MJ/kg), rice straw(17.24 MJ/kg), groundnut straw (17.10 MJ/kg), coconut shell (17.9 MJ/kg), and cotton stalk(16.23 MJ/kg) compared to other agricultural biomass residue which had calorific valuesless than 15 MJ/kg. From Table 6, it can be noticed that the high ash content of agriculturalwaste affects the heating value of briquette, which makes it less desirable as fuel, whereaslower ash content adds to its desirability [51].

Ash content is the remainder of the mass of inorganic material that remains aftercomplete burning under specific conditions, as a fraction of the original mass [52]. Biomasscombustion can form ash, and the major ash-forming elements in biomass are Na, Ca, K,Mg, and various heavy metals. Therefore, the chemical composition of biomass materialshas a significant effect on the ash content [24]. The highest ash content is for rice huskat 19.5% obtained from this study, followed by rice straw (11.25%), wheat straw (12.8%),corn stalk (10%), and other agricultural biomass that has an ash content of below 9%, asdemonstrated in Table 6.

Table 6. Comparison of agricultural biomass residue with rice husk–kraft lignin briquettes.

Crop ResidueMoistureContent

(%)

Ashcontent

(%)

VolatileMatter (%)

FixedCarbon

HHV(MJ/kg)

RelaxedDensity(kg/m3)

Ref.

Rice Straw 12.2 11.25 42.2 50.3 16.1 1011.23 [53]Rice bran 12.5 8.7 44.51 55.01 20.5 887.25 [53]Corn Stalk 16.8 10 61.20 36.4 13.79 1013.22 [54]Corn Cob 7.7 8.1 64.2 33.21 17.25 1007.32 [55]Corn Husk 11 2.84 62.98 36.15 14.44 1035.30 [56]

Wheat Straw 14.6 12.8 55.21 37.21 17.9 1022.40 [57]Oats Straw 15 na 45.2- 39.51 17.54 955.24 [58]

Barley Straw 16.3 4.3 44.52 41.21 14.85 1011.21 [58]Sorghum Straw 15 na 46.35 52.35 14.25 956.34 [31]Cassava Stalk 15 na 42.30 42.1 12.79 927.35 [59]

Groundnut Husk/Shell 7.8 na 57.1 46.43 13.15 957.25 [60]Groundnut Straw 12 1.3 43.25 39.45 17.10 834.49 [60]

Soybean Straw 15 na 48.21 47.34 14.21 954.50 [61]Sugar cane Bagasse 49.8 6 52.40 41.12 11.25 827.35 [31]Sugar cane Tops/Leaves 62.5 1.2 54.35 38.5 13.45 987.25 [62]

Cotton Stalk 12 5.4 55.21 51.3 14.23 1157 [63]Cotton Husk 10 6 49.34 13.35 1153 [64]

Coconut Shell 10.9 1 49.25 54.3 17.24 875.43 [62]Oil palm Shell 12.3 4 46.23 44.35 14.45 955.40 [65]Oil palm Fibre 36.7 5 41.34 43.2 855.59 [65]Oil palm Empty bunches 36.7 5 43.35 39.5 16.30 947.15 [65]

Coffee Husk 15 na 57.25 42.10 19.35 1132 [66]Rice Husk 10 19.5 nd 35.82 8.97 543 This study

RiceHusk blen d+kraft lignin

12 4.92 40 36.63 13.868 1136This study14 4.70 30 34.46 14.229 825

16 5.27 39 34.42 13.08 673

Rice Husk Briquette12 4.21 42 39.43 14.04 1021

This study14 3.97 40.5 41.23 17.688 80116 4.63 41 38.76 13.106 644

Sustainability 2021, 13, 3069 12 of 14

Our overall findings suggest the importance of water content in determining theproperties of the briquette product. Due to variation of the water contents of the rice huskmaterials, the findings imply the need for water content characterization and modificationof the rice husk before the briquetting process. Thanks to the desirable properties of therice husk–kraft lignin briquettes, the economic value of the agricultural biomass wastecan be enhanced and converted into a valuable product. This lightens the burden on theenvironment by reducing waste, and reducing the consumption of fossil-based fuels, whichcan be substituted by the briquettes.

4. Conclusions

Rice husk briquettes blended with kraft lignin were efficiently formed at various mois-ture contents of 12, 14, and 16%. They displayed highly durable properties, accompaniedby a good compressive strength after being forced under 5 kN without any cracking orbreaking occurring. The briquette with a moisture content of 14% possessed the standarddensity requirements for briquettes, at 825 ± 0.26 kg/m3, along with an optimum poresize. It also had the highest heating value of 17.688 MJ/Kg.

Author Contributions: Conceptualization, A.A.H.S. and N.Y.H.; methodology, N.Y.H., V.D.V. andA.A.H.S.; software, A.A.H.S. and F.A.S.R.; validation, A.A.H.S., S.A.R. and N.Y.H.; formal analysis,A.A.H.S.; investigation, A.A.H.S. and N.Y.H.; resources, N.Y.H.; data curation, A.A.H.S., H.K.A.and N.Y.H.; writing—original draft preparation, A.A.H.S. and V.D.V.; writing—review and editing,A.A.H.S., V.D.V., M.R.B. and F.A.S.R.; visualization, A.A.H.S. and N.Y.H.; supervision, N.Y.H., A.M.P.and M.T.A.; project administration, N.Y.H. and A.A.H.S.; funding acquisition, N.Y.H. All authorshave read and agreed to the published version of the manuscript.

Funding: This work was funded by the Yayasan Universiti Teknologi PETRONAS (YUTP) with grantcode YUTP-015LC0-210.

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Data Availability Statement: The data presented in this study are available from the correspondingauthor upon a reasonable request.

Acknowledgments: The authors would like to acknowledge the efforts of the Universiti TeknologiPetronas Malaysia for financing the project through the Yayasan Universiti Teknologi PETRONAS(YUTP) with grant code YUTP-015LC0-210.

Conflicts of Interest: The authors declare no conflict of interest.

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